In recent years, a wireless LAN (WLAN) employing IEEE802.11 has become popular rapidly as wireless equipment such as a smartphone and a mobile router has increased. When a plurality of wireless LAN communications are concentrated and performed close to each other, many users share an available frequency resource and hence there arises a problem that mutual communication interference occurs. In IEEE802.11, the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method is used for the purpose of suppression of interference between WLAN devices. In the CSMA/CA method, a terminal performs carrier sensing of a channel always before transmission and starts transmission after confirming the channel is not being used.
However, packet collision cannot be avoided completely by the CSMA/CA method. One reason is that even if it is determined in carrier sensing that a channel to be used for communication is available, packet collision occurs when a plurality of users start communication at the same timing. The other reason is that when the power of a transmission signal of another user is weaker than a detection threshold, the presence of the packet cannot be detected in carrier sensing. Herein, a packet is the unit of transmitted/received data handled in the data link layer of the seven layers of the OSU model.
Thus, there is a problem that it is impossible to determine whether a failure of packet transmission results from collision with an interference signal such as a packet transmitted by another terminal or results from attenuation of signal power depending on a radio wave propagation distance and it is impossible to take proper measures against transmission failures due to different causes. In view of the problems of a wireless LAN employing IEEE802.11, there is a need to detect packet collision, which is one of the causes of transmission failures, in order to take proper measures against transmission failures.
As a technique for detecting packet collision, the following technique is known (see Patent Document 1, for example). According to the technique disclosed in Patent Document 1, a transmission device transmits packets at given intervals, and a reception device detects reception power during a packet reception period and obtains an average value H1 thereof. After that, the reception device calculates a difference ΔH=(H1−H2) between the average value H1 in the current reception period and an average value H2 in the last reception period, and compares the difference ΔH with a predetermined threshold Th. In a case where the difference ΔH is equal to or more than the threshold Th, the reception device determines that packet collision is occurring in the current reception time. In a case where the difference ΔH is less than the threshold Th, the reception device determines that packet collision is not occurring.    Patent Document 1: Japanese Unexamined Patent Application Publication No. JP-A 2008-167200
In a case where transmission power in a transmission device is fixed, it is possible to correctly determine whether or not packet collision is occurring by the technique disclosed in Patent Document 1. However, according to the technique disclosed in Patent Document 1, it is determined whether or not packet collision is occurring, on the basis of whether the difference ΔH between the average value H1 of reception power in the current reception period and the average value H2 of reception power in the last reception period is equal to or more than the predetermined threshold Th. Thus, there is a problem that a current determination result cannot be obtained in a case where transmission power in a transmission device changes.
For example, in a case where transmission power in the transmission device increases and the average H1 during the current reception period increases by the threshold Th or more from the average value H2 during the last reception period, it is determined that packet collision is occurring even if packet collision is not occurring. Moreover, in a case where transmission power in the transmission device decreases, it may be determined that packet collision is not occurring though packet collision is occurring. These problems occur in the case of using a transmission device which has a power control function of switching transmission power on the basis of the strength of a signal transmitted from the other party of communication, a packet loss rate and so on, or a transmission device which has a function of switching transmission power in accordance with a setting operation by the user.